Chromium-containing "bonding coating" compositions for metal substrates that are applied prior to painting typically impart corrosion resistance to the surface of the substrate. Such compositions, when applied and cured on a metal substrate, exhibit excellent adherence to the substrate and offer many further desirable characteristics. These include substrate corrosion resistance, the advantage of metal formability without significant coating loss, and retention of substrate weldability. Much of the early efforts with bonding coatings involved developing combinations of hexavalent-chromium-containing substances, often supplied by chromic acid, with a wide variety of reducing agents. For example, U.S. Pat. No. 3,779,815 teaches the utility of amino acids, or of compounds containing the structure -CONH-, as reducing agents. Further, U.S. Pat. No. 3,382,081 discloses combinations of organic reducing agents that can include aliphatic dicarboxylic acids in the combination.
Coating characteristics including corrosion resistance can be augmented by including in the pre-paint coating composition a particulate metal, such a finelydivided zinc. For example, in U.S. Pat. No. 3,671,331 there are disclosed pulverulent metal-containing chromium bonding coating compositions. These contain, in addition to pulverulent metal, the hexavalent-chromium-containing substance plus reducing agent for such substance.
Because such compositions must perform a wide variety of functions, and perform them well, it has been found to be extremely difficult to upgrade a particular coating characteristic without downgrading some others. Or without downgrading the coating composition itself, as by retarding shelf life. It would be most highly desirable to enhance corrosion resistance, for example, while maintaining or improving top coat adhesion, weldability of the coated substrate and formability.
It has more recently been shown, as in U.S. Pat. No. 3,990,920, that desirable coating characteristics for these bonding coating compositions containing particulate metal, can be improved by exercising control over the pH of the coating composition. This is done by introducing a pH adjusting agent into the composition. On the one hand, the agent can enhance compositional stability. On the other hand, these formulations are becoming increasingly complex. So, desirable features must not only include those of the coating, but also those of the composition, e.g., extended compositional stability.
Improvements in coating characteristics have thus been sought without sacrifice to the desirable features of the composition itself. In such efforts, the complexity of the composition must be taken into careful consideration.